1. Field of the Invention
This document relates to a liquid crystal display and a method for driving the same.
2. Discussion of the Related Art
Flat panel displays include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), an organic light emitting display (OLED), etc.
Since the LCD satisfies the trend toward lightweight, thin, short and small electric appliances and has improved mass productivity, cathode ray tubes have been rapidly replaced with LCDs in many applications. An active matrix type LCD which drives liquid crystal cells using thin film transistors (hereinafter, referred to as “TFTs”) has excellent picture quality and low power consumption, has been rapidly developed to realize an increase in size and a high resolution by a recent mass production technology and the results of research and development, and has been quickly replacing cathode ray tubes in television sets (hereinafter, TVs) and monitors. However, the LCD has a relatively low response speed due to characteristics such as the inherent viscosity and elasticity of liquid crystal, as can be seen from the following equations 1 and 2:
                              τ          T                ∝                              γ            ⁢                                                  ⁢                          d              2                                            Δɛ            ⁢                                                                          V                  a                  2                                -                                  V                  F                  2                                                                                                      [                  Equation          ⁢                                          ⁢          1                ]            
where τr is a rising time when a voltage is applied to the liquid crystal, Va is the applied voltage, VF is a Freederick transition voltage at which liquid crystal molecules start to be inclined, d is a liquid crystal cell gap, and γ is the rotational viscosity of the liquid crystal molecules.
                              τ          f                ∝                              γ            ⁢                                                  ⁢                          d              2                                K                                    [                  Equation          ⁢                                          ⁢          2                ]            
where τf is a falling time when the liquid crystal is returned to its original position after the voltage applied to the liquid crystal is turned off, and K is the inherent elastic modulus of the liquid crystal.
In a twisted nematic (TN) mode which has been mostly commonly used in liquid crystal displays, although the response speed of the liquid crystal may be different according to the physical properties and cell gap of the liquid crystal, it is common that the rising time is 20 ms to 80 ms and the falling time is 20 ms to 30 ms. Because this liquid crystal response speed is longer than one frame period (16.67 ms in National Television Standards Committee (NTSC)), the response of the liquid crystal proceeds to the next frame before a voltage being charged on the liquid crystal reaches a desired level, as shown in FIG. 1, resulting in motion blurring in a moving image.
With reference to FIG. 1, a liquid crystal display device cannot express a desired color and brightness in that, when data VD is changed from one level to another level, the corresponding display brightness level BL is unable to reach a desired value due to the slow response of the liquid crystal. As a result, motion blurring occurs in the moving image, causing degradation in display quality.
In order to solve the low response speed of the liquid crystal display, U.S. Pat. No. 5,495,265 and PCT International Publication No. WO 99/05567 have proposed a method for modulating data according to a variation therein using a look-up table (referred to hereinafter as an ‘over-driving method (ODC)’). This over-driving method is adapted to modulate data on the basis of a principle as illustrated in FIG. 2.
With reference to FIG. 2, the over-driving method includes modulating an input data voltage VD to a preset modulated data voltage MVD and applying the modulated data voltage MVD to a liquid crystal cell to obtain a desired brightness level MBL. In this over-driving method, in order to obtain the desired brightness level in one frame period, V2a−V2F of Equation 1 is increased on the basis of a variation in the input data. Accordingly, a liquid crystal display using the over-driving method is able to compensate for a slow response of a liquid crystal by modulation of a data value to reduce motion blurring in a moving image.
In this over-driving method, data of a previous frame and current frame are compared with each other, and if there is any change between the data, the data of the current frame is modulated to a preset modulation data
FIG. 3 is a block diagram schematically showing an over-driving circuit.
Referring to FIG. 3, the over-driving circuit includes first and second frame memories 33a and 33b for storing data from a data input bus 32 and a look-up table 34 for modulating data.
The first and second frame memories 33a and 33b alternately store the data input from the data input bus 32 in a frame unit in accordance with a pixel clock, and then alternately output the stored data to supply a previous frame data, i.e., (n−1)th frame data Fn−1 to the look-up table 34.
The look-up table 34 selects a preset modulation data MRGB from the following Table 1 by using the (n) th frame data Fn and the (n−1)th frame data Fn−1 from the first and second frame memories 33a and 33b as the address, thereby modulating the data. The lookup table 34 includes a read only memory ROM and a memory control circuit.
TABLE 101234567891011121314150023456791012131415151515101345678101213141515151520024567810121314151515153001356781011131415151515400134678911121314151515500123578911121314151515600123468910121314151515700123457910111314151515800123456810111214151515900123456791112131415151000123456781012131415151100123456789111314151512001234567891012141515130012334567810111315151400123345678911121415150001233456789111315
In the Table 1, the leftmost column represents the data of the previous frame Fn−1 and the uppermost row represents the data of the current frame Fn.
During an (n) th frame period, as represented by a solid line, the (n)th frame data Fn is stored in the first frame memory 33a and supplied to the look-up table 34 in accordance with the same pixel clock. At the same time, for a (n)th frame period, the second frame memory 33b supplies the (n−1)th frame data Fn−1 to the look-up table 34.
On the other hand, for a (n+1)th frame period, as represented by a dotted line, the current (n+1)th frame data Fn+1 is stored in the second frame memory 33b and simultaneously supplied to the look-up table 34 in accordance with the same pixel clock. At the same time, for a (n+1)th frame period, the first frame memory 33a supplies the (n) th frame data Fn to the look-up table 34.
However, although the over-driving method may enhance motion picture response time further by increasing over/under shoot ratio, picture quality may be degraded due to motion blurring occurring at a liquid crystal charging voltage.